WFPC2 images of the central regions of early-type galaxies. I. The data

We present high-resolution R-band images of the central regions of 67 early -type galaxies obtained with the Wide Field and Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). This homogeneously selected sample roughly doubles the number of early-type galaxies that have now been image d at HST resolution and complements similar data on the central regions of radio galaxies and the bulges of spiral galaxies. Our sample strikingly con firms the complex morphologies of the central regions of early-type galaxie s which have become apparent from previous studies with HST. In particular, we detect dust, either in the form of nuclear disks or with a filamentary distribution, in 43% of all galaxies, in good agreement with previous estim ates. In addition, we find evidence for embedded stellar disks in a remarka bly large fraction of 51%. In 14 of those galaxies the disklike structures are misaligned with the main galaxy, suggesting that they correspond to ste llar bars in S0 galaxies. We analyze the luminosity profiles of the galaxie s in our sample and classify galaxies according to their central cusp slope . To a large extent we confirm the results from previous HST surveys in tha t early-type galaxies reveal a clear dichotomy: the bright ellipticals (M-B less than or similar to -20.5) are generally boxy and have luminosity prof iles that break from steep outer power laws to shallow inner cusps (referre d to as "core" galaxies). The fainter ellipticals, on the other hand, typic ally have disky isophotes and luminosity profiles that lack a clear break a nd have a steep central cusp (referred to as "power-law" galaxies). The adv antages and shortcomings of classification schemes utilizing the extrapolat ed central cusp slope gamma are discussed, and it is shown that gamma might be an inadequate representation for galaxies whose luminosity profile slop e changes smoothly with radius rather than resembling a broken power law. T hus, we introduce a new, alternative parameter and show how this affects th e classification. In fact, we find evidence for an "intermediate" class of galaxies that cannot unambiguously be classified as either core or power-la w galaxies and that have central cusp slopes and absolute magnitudes interm ediate between those of core and power-law galaxies. It is unclear at prese nt, however, whether these galaxies make up a physically distinct class or whether distance and/or resolution effects cause them to lose their distinc t core or power-law characteristics.